Filter medium

ABSTRACT

The filter fabric containing polyphenylene sulfide fibers, characterized in that the following (1) and/or (2) is satisfied is used to provide a filter fabric and a bag filter composed of the filter fabric excellent in dust-collecting efficiency and high in mechanical strength: (1) Containing at least two webs, one of which on the dust side contains 50 wt % or more of heat resistant fibers with a fiber, diameter of 15 μm or less, while the other web on the clean side contains 50 wt % or more of heat resistant fibers with a fiber diameter of 20 μm or more (2) Containing a web containing polyphenylene sulfide staple fibers with their fineness kept in a range from 1 to 3 dtex and fluorine-based staple fibers with their fineness kept in a range from 2 to 4 dtex.

TECHNICAL FIELD

The present invention relates to a filter fabric that can function tocollect and remove contaminants such as dust in air for cleaning theair. The filter fabric of this invention can be suitably used as adust-collecting filter cloth for filtering the high temperature exhaustgases emitted from refuse incinerators, coal boilers, metal blastfurnaces and the like, and also as a bag filter produced by sewing saidfilter cloth.

BACKGROUND ART

Filter fabrics used for cleaning air include filter fabrics for depthfiltration and filter fabrics for surface filtration, and dustcollectors use the filter fabrics for surface filtration. In the case ofsurface filtration, dust is collected on the surface of a filter fabric,to form a dust layer on the surface of the filter fabric, and dust issuccessively collected by the dust layer. When the dust layer grows tohave a certain thickness, it is removed from the surface of the filterfabric by means of air pressure, and the operation to form a dust layeron the surface of the filter fabric is repeated again.

The fibers constituting the filter fabrics used for filtering the hightemperature exhaust gases emitted from refuse incinerators, coalboilers, metal blast furnaces and the like include polyphenylene sulfide(hereinafter abbreviated as PPS) fibers, meta-aramid fibers,fluorine-based fibers, polyimide fibers, etc. respectively excellent inheat resistance and chemical resistance, and they are formed intononwoven fabrics to be used as filter fabrics. Among them, PPS fibersare excellent in hydrolytic resistance, acid resistance and alkaliresistance, and widely used as bag filters for dust collection of coalboilers.

Meanwhile, environmental regulations tend to be more severe in theworld, and especially the PM2.5 regulations that are going to be enactedin USA may be applied also in Japan. To meet this situation, it isdemanded that a filter with a higher dust-collecting efficiency andexcellent dimensional stability at higher temperatures is available. Theconventional electrostatic precipitators and cyclone separators cannotcatch up with the demand, and it is desired that a nonwoven fabricfilter with higher functions is available.

Moreover, when such filter fabrics are used, for example, in refuseincinerators, the chemical deterioration caused by high temperatureexhaust gases and the chemicals, etc. contained in the exhaust gasestakes place, and at the same time, in addition, the physicaldeterioration such as the wearing due to the contact with the cage andthe flexing fatigue respectively caused by the pressure loss duringexhaust gas filtration and by the pulse jetting during back-pulsecleaning also takes place. Therefore, the filter fabrics used in bagfilters are required to have mechanical strength such as abrasionresistance, in addition to the above-mentioned heat resistance,chemicals resistance and hydrolytic resistance.

JP10-165729A proposes a filter cloth in which PPS fibers with a singlefiber fineness of 1.8 d (2.0 dtex) or less are disposed as the surfacelayer. This method is certainly good in dust release characteristics anddust collection performance, but since it is insufficient in thestiffness at high temperature and abrasion resistance, it has such aproblem that the filter cloth is physically progressively deterioratedand broken while it is used.

Furthermore, JP9-075637A proposes a filter cloth for bag filters,comprising a felt composed of fluorine staple fibers, in which thestaple fibers in an upper layer portion are different in diameter fromthose in a lower layer portion. In this invention, a layer composed offine fibers is disposed on the upstream side while thick fibers aredisposed on the downstream side, to provide a filter cloth gentlyincreasing in pressure loss, hence having a longer life. However, sincefluorine fibers are relatively low in stiffness, there is a fear thatthe stiffness especially at high temperature declines. Furthermore,since fluorine fibers are insufficient in abrasion resistance, there issuch a problem that the filter cloth is broken due to physicaldeterioration during use.

Moreover, in order to enhance the mechanical strength of a filter,JP7-16570B proposes a filter, in which a very fine fiber layer and afelt substrate layer are integrated by means of needle punchingtreatment to gradually decrease the distribution of the fibers capableof being made very fine, from the front surface to the back surface, andsubsequently high pressure water jet punching is used to divide thefibers capable of being made very fine, to make the fibers very fine.This method can certainly enhance the dust-collecting efficiency owingto the very fine fibers of the felt surface layer, but has a problemthat increased processing steps raise the processing cost.

Moreover, JP2000-334228A proposes a heat resistant filter cloth formedby laminating and integrating a lap consisting of, for example,polytetrafluoroethylene fibers and PPS fibers, a woven fabric composedof PPS fibers and a woven fabric composed of glass fibers in this order.This method is intended to enhance the mechanical strength by laminatinga woven fabric composed of glass fibers, but has such problems thatsince the glass fibers are low in the resistance against alkalinechemicals, a heat resistant filter cloth using the chemical resistanceof PPS fibers cannot be provided and that the strength decline duringwet heat treatment (autoclave treatment) is very large.

JP2000-140530A proposes a filter cloth for high performance bag filters,obtained by blending, for example, PPS fibers and at least one or morekinds of fibers selected from polyimide fibers, polyamideimide fibers,polytetrafluoroethylene fibers and glass fibers. However, this inventionuses PPS fibers with a 180° C. dry heat shrinkage rate of 3% or more,and does not improve the dimensional stability of a nonwoven fabricfilter using PPS fibers. Furthermore, the filter cloth has a problemthat when PPS fibers and other fibers are blended, blending irregularityis likely to occur.

JP2002-204909A proposes a filter cloth in which fluorine fibers areentangled with the surface of a heat resistant substrate. This method iscertainly good in dust release characteristics, in preventing thepenetration of particles into the filter cloth and in reducing thepressure loss during the operation of the dust collector. However, thefilter cloth has such problems that since the air permeability in theinitial state is so low as to increase the initial pressure loss, thelife of the filter cloth is shortened, and that the capability oftreating the exhaust gas greatly declines. Moreover, it has a problemthat plural processing steps of laminating a web composed of staplefibers of polytetrafluoroethylene and entanglement treatment arenecessary after preparation of a heat resistant felt substrate.Furthermore, it has a problem that the laminated web layer composed ofstaple fibers of polytetrafluoroethylene is separated by impact duringuse as a bag filter.

DISCLOSURE OF THE INVENTION

In view of the above-mentioned technical background, the presentinvention provides a filter fabric excellent in dust-collectingefficiency, small in the rise of pressure loss after pulse cleaning andhigh in mechanical strength.

Furthermore, the present invention provides a dense filter fabricexcellent in dust-collecting efficiency, excellent in thermaldimensional stability at high temperature and uniform in the fabricuniformity.

This invention employs the following means for solving theabove-mentioned problems. That is, the filter fabric of this inventionis a filter fabric containing polyphenylene sulfide fibers,characterized in that the following (1) and/or (2) is satisfied:

-   (1) Containing at least two webs, one of which on the dust side    contains 50 wt % or more of heat resistant fibers with a fiber    diameter of 15 μm or less, while the other web on the clean side    contains 50 wt % or more of heat resistant fibers with a fiber    diameter of 20 μm or more-   (2) Containing a web containing polyphenylene sulfide staple fibers    with their fineness kept in a range from 1 to 3 dtex and    fluorine-based staple fibers with their fineness kept in a range    from 2 to 4 dtex.

This invention includes a bag filter formed by sewing said filter fabriccylindrically.

The first invention provides a filter fabric containing polyphenylenesulfide fibers, comprising at least two webs, one of which on the dustside contains 50 wt % or more of heat resistant fibers with a fiberdiameter of 15 μm or less, while the other web on the clean sidecontains 50 wt % or more of heat resistant fibers with a fiber diameterof 20 μm or more. Therefore, the filter fabric is excellent indust-collecting efficiency, small in the rise of pressure loss afterpulse cleaning and excellent in mechanical strength.

Furthermore, the second invention provides a filter fabric containingpolyphenylene sulfide fibers, comprising a web containing polyphenylenesulfide staple fibers with their fineness kept in a range from 1 to 3dtex and fluorine-based staple fibers with their fineness kept in arange from 2 to 4 dtex. Therefore, the filter fabric is excellent indust-collecting efficiency, excellent in thermal dimensional stabilityat high temperature, uniform in the fabric uniformity, and having fewdefects such as pinholes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded sectional view showing the filter fabric of thisinvention as an example.

FIG. 2 is a schematic drawing showing an instrument for measuring thedust-collecting efficiency of the filter fabric of this invention fordust in the air.

FIG. 3 is a schematic drawing showing an instrument for measuring thepressure loss of the filter fabric of this invention after pulsecleaning.

FIG. 4 shows the results of measuring the dust-collecting efficienciesof the filter fabrics of this invention for dust in the air at an airvelocity of 1 m/min.

FIG. 5 shows the results of measuring the pressure losses of the filterfabrics of this invention after pulse cleaning at an air velocity of 2m/min.

MEANINGS OF SYMBOLS

-   1: web forming the filtration layer of dust side-   2: woven fabric composed of heat resistant fibers (scrim)-   3: web forming the filtration layer of clean side-   4: particle counter (upstream)-   5: filter fabric-   6: particle counter (downstream)-   7: manometer-   8: air blower-   9: pulse jet loading machine-   10: flow meter-   11: dust collection filter-   12: vacuum pump-   13: digital manometer-   14: dust feeder-   15: dust dispersing device-   16: box for collecting the dust removed by pulse cleaning-   17: atmospheric dust-containing air-   18: air remaining after atmospheric dust removal-   19: metered dust-   20: dust-containing air-   21: air remaining after dust removal

THE BEST MODES FOR CARRYING OUT THE INVENTION

The inventors studied intensively filter fabrics, and found that afilter fabric containing polyphenylene sulfide fibers, which satisfiesthe following (1) and/or (2), has excellent properties:

-   (1) Containing at least two webs, one of which on the dust side    contains 50 wt % or more of heat, resistant fibers with a fiber    diameter of 15 μm or less, while the other web on the clean side    contains 50 wt % or more of heat resistant fibers with a fiber    diameter of 20 μm or more-   (2) Containing a web containing polyphenylene sulfide staple fibers    with their fineness kept in a range from 1 to 3 dtex and    fluorine-based staple fibers with their fineness kept in a range    from 2 to 4 dtex.

That is, the first invention provides a filter fabric containingpolyphenylene sulfide fibers, comprising at least two webs, one of whichon the dust side contains 50 wt % or more of heat resistant fibers witha fiber diameter of 15 μm or less, while the other web on the clean sidecontains 50 wt % or more of heat resistant fibers with a fiber diameterof 20 μm or more.

Furthermore, the second invention provides a filter fabric containingpolyphenylene sulfide fibers, comprising a web containing polyphenylenesulfide staple fibers with their fineness kept in a range from 1 to 3dtex and fluorine-based staple fibers with their fineness kept in arange from 2 to 4 dtex.

Moreover, it is more preferred that both (1) and (2) are satisfied. Atfirst, the first invention is explained below.

The polyphenylene sulfide (hereinafter abbreviated as PPS) fibers usedin this invention are fibers made of a polymer, in which 90% or more ofits component units are phenylene sulfide structural units representedby —(C₆H₄—S)—. If the PPS fibers are used, a filter fabric excellent inheat resistance, chemical resistance and hydrolytic resistance can beobtained.

The filter fabric containing the PPS fibers of this invention containsat least two webs, one of which on the dust side contains 50 wt % ormore of heat resistant fibers with a fiber diameter of 15 μm or less,while the other web on the clean side contains 50 wt % or more of heatresistant fibers with a fiber diameter of 20 μm or more. It is notpreferred that the web on the dust side contains more than 50 wt % ofheat resistant fibers with a fiber diameter of more than 15 μm, sincethe dust-collecting efficiency as a property of filter performance tendsto decline owing to poor denseness. Furthermore, it is not preferredeither that the web on the clean side contains more than 50 wt % of heatresistant fibers with a fiber diameter of less than 20 μm, since themechanical strength of the filter fabric, if consisting of the web only,tends to decline.

In this invention, the dust side refers to the face on which thedust-containing air contacts the filter fabric for the first time whenthe filter fabric is used for surface filtration. That is, it means thefilter fabric surface on which dust is collected to form a dust layer.Furthermore, the face on the other side, namely, the face from which theair remaining after dust removal goes out, is defined as the clean side.

In the filter fabric of this invention, since the web on the clean sidecontains 50 wt % or more of heat resistant fibers with a fiber diameterof 20 μm or more, the filter fabric obtained can be excellent inmechanical strength such as dimensional stability and tensile strength.Since a nonwoven fabric obtained by entangling a web has void portionsuniformly dispersed and excellent filtration properties compared with awoven fabric, it can be preferably used as a filter fabric. However, anonwoven fabric formed from a web consisting of thin fibers only is notpreferred, since it is low in tensile strength and dimensionalstability, being insufficient in mechanical strength.

As the constitution of the heat resistant fibers constituting the filterfabric, in view of the balance among dust-collecting efficiency,pressure loss and mechanical strength, it is preferred that the web onthe dust side contains 50 wt % or more of heat resistant fibers with afiber diameter of 9 to 15 μm. Furthermore, it is preferred that the webon the clean side contains 50 wt % or more of heat resistant fibers witha fiber diameter of 20 to 40 μm. In the case where fibers other than theheat resistant fibers with a fiber diameter of 9 to 15 μm are blended asheat resistant fibers forming the web on the dust side, it is preferredthat the web contains 50 wt % or less of heat resistant fibers with afiber diameter of more than 15 μm to 40 μm. In the case where fibersother than the heat resistant fibers with a fiber diameter of 20 to 40μm are blended as heat resistant fibers forming the web on the cleanside, it is preferred that the web contains 50 wt % or less of heatresistant fibers with a fiber diameter of 9 μm to less than 20 μm.

Furthermore, it is preferred that the heat resistant fibers forming saidweb on the dust side contain 20 wt % or more of heat resistant fiberswith a fiber diameter of 10 μm or less, since a higher dust-collectingefficiency can be obtained. It is more preferred that they contain 20 wt% or more of heat resistant fibers with a fiber diameter of 9 μm to 10μm.

In this invention, the heat resistant fibers forming a web contain atleast PPS fibers, but can also have heat resistant fibers other than PPSfibers blended. The dust-collecting efficiency and the pressure loss asproperties of filter performance are contradictory to each other. Ingeneral, the means for enhancing the dust-collecting efficiency includecoating the filtration layer on its surface with a resin such assilicone resin or fluorine resin, to form a film, and using fibers witha smaller fineness as the fibers constituting the filtration layer.However, if such means are employed, the pressure loss tends to behigher, thereby shortening the life as a bag filter. If the fibersdifferent in the property of being electrified are blended, an electricaction works between fibers (triboelectric effect), allowing thedust-collecting efficiency to be enhanced. So, this is effective forachieving a good balance between the dust-collecting efficiency and thepressure loss. For this reason, it is preferred that especially the webon the dust side has heat resistant fibers other than PPS fibersblended. It is of course preferred that the web on the clean side hasalso heat resistant fibers other than PPS fibers blended.

As the heat resistant fibers other than PPS fibers, preferred are fibersselected from fluorine-based fibers, para-aramid fibers, meta-aramidfibers, polyimide fibers, carbon fibers and glass fibers. Especiallyfluorine-based fibers are preferred, since they have heat resistancehigher than that of PPS fibers and are also excellent in chemicalresistance. Furthermore, if the web on the dust side containsfluorine-based fibers, the low surface friction property of thefluorine-based fibers acts to allow the dust deposited on the surfacesof the fibers to be removed by pulse cleaning easily, and the rise ofpressure loss can also be inhibited. Moreover, since the penetration anddeposition of dust into the web can also be inhibited, the rise ofpressure loss can also be inhibited similarly. As the fluorine-basedfibers, fibers of any polymer, 90% or more of the recurring structuralunits of which are formed by a monomer having one or more fluorine atomson its main chain or side chain, can be used. Fibers formed from amonomer having more fluorine atoms are more preferred. Examples of thepolymer include tetrafluoroethylene-hexafluoropropylene copolymer (FEP),tetrafluoroethylene-fluoroalkylvinylether copolymer (PFA),ethylene-tetrafluoroethylene copolymer (ETFE), polytetrafluoroethylene(PTFE), etc. It is further preferred to use polytetrafluoroethylene(PTFE) as the fluorine-based fibers, since it is especially excellent inheat resistance, chemical resistance and low surface friction property.

It is preferred that each of the webs of this invention contains 10 wt %or more of PPS fibers. If the content is less than 10 wt %, the rate ofheat resistant fibers other than PPS fibers is too large. So, theproperties of PPS fibers per se maybe impaired depending on the selectedfibers. Furthermore, in the case where fibers other than PPS fibers areblended, it is preferred that the content of PPS fibers in the web is 90wt % or less. If the content is more than 90 wt %, the rate of heatresistant fibers other than PPS fibers becomes very low, and the effectof the other fibers blended in enhancing the dust-collecting efficiencybecomes low.

That is, in the case where PPS fibers and fluorine-based fibers areblended, it is preferred that the web contains 10 to 90 wt % of PPSfibers and 10 to 90 wt % of fluorine-based fibers. Still furthermore, itis preferred that the total rate of PPS fibers and fluorine-based fibersin a web as a whole is 70 wt % or more. It is not preferred that thetotal rate of PPS fibers, and fluorine-based fibers in the entirenonwoven fabric is less than 70 wt %, since the retention of strengthafter treatment at 160° C. for 200 hours in an autoclave, i.e., heatresistance declines.

The PPS fibers used in this invention can be produced by heatingphenylene sulfide polymer at a temperature higher than its meltingpoint, for melt spinning into fibers, passing them through heated steamor hot bath, drawing and crimping. PPS staple fibers can be obtained byfurther cutting them to a desired length. In the drawing step, drawingin a relaxed state is preferred to drawing in a tensioned state, sincethe fibers can be stably passed through the step with few occurrences offiber breaking, etc. Therefore, usually, drawing in a relaxed state isemployed, but in this case, the Young's modulus of the PPS fibersbecomes low. In this invention, heat treatment under tension is carriedout in the drawing step, so that PPS fibers with a Young's modulus of 20cN/dtex or more can be obtained. PPS fibers with a Young's modulus of 20cN/dtex or more can be especially suitably used, since the dimensionalstability of the filter fabric can be improved. This is especiallypreferred in the case where the filter fabric is used as a bag filter,since the dimensional stability against the impact during back-pulsecleaning by pulse jetting and against the stress due to the self weightof the dust deposited on the surface of the filter fabric can beimproved. Furthermore, if PPS fibers with a Young's modulus of 20cN/dtex or more are used, the mechanical strength of the filter fabriccan be secured even in the case where fluorine-based fibers lower inmechanical strength than PPS fibers are blended.

The polytetrafluoroethylene fibers used in this invention can beproduced, for example, by an emulsion spinning method comprising thesteps of mixing a matrix polymer and tetrafluoroethylene polymer to forman emulsion, and discharging said emulsion from a molding die into acoagulating bath for forming fibers (also called a wet emulsion spinningmethod or matrix spinning method). The polytetrafluoroethylene fiberscan also be obtained without any problem by a paste extrusion method inwhich a paste obtained by mixing a plasticizing aid such as solventnaphtha with tetrafluoroethylene polymer is extrusion-molded, or askiving method in which a heated molding of tetrafluoroethylene isskived into films, followed by splitting, or direct separation intofilaments which are then burned and drawn into fibers.

In this invention, each of the webs can also contain glass fibers. Glassfibers are inferior to PPS fibers in chemical resistance (alkaliresistance), but are high in heat resistance and low in cost. Glassfibers are artificial fibers obtained by drawing glass thinly, and sincemolten glass is drawn through numerous pores at high speed for beingspun, fibers with a very small fineness can be obtained. So, they arevery effective for enhancing the dust-collecting efficiency. It ispreferred that the fiber diameter of glass fibers is in a range from 2to 7 μm in view of the balance between the dust-collecting efficiencyand the pressure loss as properties of filter performance.

As the method for producing the filter fabric of this invention, forexample, a method comprising the steps of producing a web containing 50wt % or more of heat resistant fibers with a fiber diameter of 15 μm orless as the web on the dust side, further producing a web containing 50wt % or more of heat resistant fibers with a fiber diameter of 20 μm ormore as the web on the clean side, and entangling and integrating boththe webs can be suitably used. As the method for producing a web, amethod in which heat resistant staple fibers, are passed through acarding machine to form a web can be suitably used. Furthermore, as themethod for entangling and integrating webs, needling punching or waterjet punching is preferred.

A generally known needle punching step is explained below. At first, rawfibers are fed onto a rotary drum cylinder having innumerable needlesfor paralleling fibers in a certain direction (carding step), and theobtained web is lapped in alternating directions at a certain rate on alattice by means of a cross lapper. It can be said that the weight perunit area of the finally finished filter fabric is virtually decided bythe quantity of the raw fibers fed in this case and the line speed. If alarge quantity of raw fibers is fed at a low line speed, the weight perunit area of the web tends to be large, and if a small quantity of rawfibers is fed at a high line speed, the weight per unit area of the webtends to be small. The obtained web is compressed lightly by means ofpressing rolls, to form a lap, and a needle punch is used to entanglethe fibers in the thickness direction, for forming the web on the dustside or clean side.

It is preferred that the needling density of said needle punch is 300needles/cm² or more in view of the strength, apparent density and airpermeability of the filter fabric. It is not preferred that the needledensity is too low, for such reasons that the fibers can be only weaklyentangled with each other to lower the strength of the filter fabric,that since the apparent density also tends to be low, the filter fabricis so loosely meshed that the dust collection performance may becomepoor. It is not preferred either that the needle density is too large onthe contrary, for such reasons that since the fibers are flawed by theneedles, the strength of the filter fabric may decline, and that sincethe filter fabric more tends to shrink, the air, permeability becomeslow to raise the pressure loss from the initial state of use forshortening the life, though the dust collection performance can beimproved because of a higher apparent density.

From the above, it is preferred that the apparent density of the filterfabric is in a range from 0.1 to 1.5 g/cm³ by adequately adjusting theneedle punching condition. A more preferred range is from 0.1 to 0.6g/cm³. A filter fabric with an apparent density of lower than 0.1 g/cm³is not preferred, since the quantity of dust that cannot be collectedincreases. A filter fabric with an apparent density of higher than 0.6g/cm³ is not preferred either, since the air permeability is so small asto greatly raise the pressure loss when the filter fabric is used as afilter. It is also preferred that the air permeability is kept in arange from 10 to 80 cc/cm²/sec by adequately adjusting the needlepunching condition.

It is further preferred that the filter fabric of this invention has anat least three-layer structure having a web on the dust side laminatedon one surface of a woven fabric (hereinafter called an scrim) composedof heat resistant fibers and having another web on the clean side on theother surface of the woven fabric. The filter fabric of this inventionhas sufficient mechanical strength even if it does not contain any scrimin a scrim-less constitution, in the case where the web on the cleanside contains 50 wt % or more of heat resistant fibers with a fiberdiameter of 20 μm or more. However, if the filter fabric has thethree-layer structure, it can be more excellent in mechanical strengthsuch as dimensional stability, tensile strength and abrasion resistance,and also excellent in dust-collecting efficiency. Moreover, in the casewhere the filter fabric is used as a bag filter, the wear of the filterfabric due to the contact with the cage can be reduced. In this case,the cage refers to a cylindrical skeleton used to be covered with a bagfilter, and it is generally made of a metal. Owing to filtrationpressure and the pulse jet during back-pulse cleaning, the bag filtercontacts the cage and is worn. If the three-layer structure is employed,the web on the clean side shows an effect of reducing the wear of thefilter fabric. Especially in this invention, it is preferred that theweb on the clean side contains 50 wt % or more of heat resistant fiberswith a fiber diameter of 20 μm or more, since the wear of the filterfabric due to the contact with the cage can be greatly reduced.

Since the scrim serves to hold mechanical strength, a scrim with thetensile strength kept in a range from 350 to 900 N/5 cm can be suitablyused.

In this invention, the fibers constituting the scrim are only requiredto have heat resistance, and the fibers that can be used forconstituting the scrim include para-aramid fibers, meta-aramid fibers,PPS fibers, polyimide fibers, fluorine-based fibers, carbon fibers,glass fibers, etc. Among them, in view of chemical resistance andhydrolytic resistance, it is preferred to use fibers selected from PPSfibers and fluorine-based fibers. As the fluorine-based fibers, thoseenumerated in the above explanation concerning webs can be used. PPSfibers are most preferred, since they have high mechanical strength.Furthermore, though fluorine-based fibers are inferior in mechanicalstrength to PPS fibers, it is preferred to use fluorine-based fiberswhen the filter fabric is used in an especially severe environment,since they have excellent heat resistance and chemical resistance.

As the fibers constituting the scrim, it is preferred to use spun yarnsor multifilaments. Especially spun yarns can be more suitably used, forsuch reasons that they can be well entangled with the web, and thatsince they have a large surface area, the dust-collecting efficiency ofthe filter fabric is good.

The fineness of the fibers constituting the scrim is not especiallylimited, if the fibers have adequate strength. It is not preferred thatthe fineness is too large, since the meshes of the scrim tend to beclosed depending on weaving conditions, to raise the pressure loss. Itis not preferred either that the fineness is too small on the contrary,since the strength of the scrim per se declines to lower the mechanicalstrength of the filter fabric, though the weaving density declines toraise the air permeability, giving a tendency of lowering the pressureloss. It is preferred that the total fineness as yarns of the fibersconstituting the scrim is in a range from 100 to 1000 dtex. A morepreferred range is from 300 to 600 dtex. If the total fineness is lessthan 100 dtex, the effect of enhancing the dimensional stability andtensile strength by the lamination of the scrim cannot be sufficientlyobtained. Furthermore, it is not preferred that the total fineness ismore than 1000 dtex, for such a reason that since the air permeabilityof the filter fabric tends to be small, the initial pressure lossbecomes high to shorten the life in the case where the filter fabric isused as a bag filter, though the dust-collecting efficiency as aproperty of filter performance is good, while the dimensional stabilityand tensile strength are excellent.

Especially it is preferred to use spun yarns of PPS staple fibers with aYoung's modulus of 20 cN/dtex or more. PPS staple fibers with a Young'smodulus of 20 cN/dtex or more are preferred, since they are excellent indimensional stability.

It is preferred that the scrim has a coarsely meshed weave texture, lestthe pressure loss as a property of filter performance should beaffected. Usable general structures include plain weave, double weave,triple weave, twill weave, satin weave, etc. Especially a generalpurpose plain weave available at low cost can be preferably used, sincea filter fabric with satisfactory performance can be obtained. As forthe weaving density, it is preferred that the warp density is in a rangefrom 15 to 40 threads/2.54 cm. A more preferred range is from 20 to 30threads/2.54 cm. It is preferred that the weft density is in a rangefrom 10 to 30 threads/2.54 cm. A more preferred range is from 15 to 25threads/2.54 cm.

After the web on the dust side, the scrim and the web on the clean sideare laminated in this order, they are entangled for integration. As theentangling means, at least one means selected from needle punching andwater jet punching is preferred. In view of entanglement strength, it ispreferred to employ needle punching, but depending on the requiredpressure loss and dust collection performance, water jet punching may bepreferred as the case may be. Furthermore, using these means forcombined treatment may provide a well-balanced filter fabric as the casemay be. So, it is preferred to adequately select and employ the means.

In the filter fabric of this invention, if the web surface on the dustside where dust is deposited is partially fused, the dust releasecharacteristics and the dust-collecting efficiency can be enhanced. Asthe method for partially fusing the web surface, such a method assingeing treatment or calendering processing can be used. Especially inthe case where a filter fabric with a high dust-collecting efficiency isdemanded, a filter fabric treated on both the web surfaces can bepreferably used. Particularly, singeing treatment is applied to the dustside of the filter medium by means of a burner flame, infrared heater orthe like, or a hot roll is used to press the dust side. Such a treatmentcauses the web surface of the dust side to be partially fused or to haveits meshes closed, or further both the means can be used forcalendering, to enhance the dust-collecting efficiency.

The second invention is explained below. It is important that the filterfabric of this invention has a constitution comprising a web containingPPS staple fibers with their fineness kept in a range from 1 to 3 dtexand fluorine-based staple fibers with their fineness kept in a rangefrom 2 to 4 dtex. This constitution, in which the two kinds of fiberswith their diameters kept close to each other in a range from about 9 toabout 15 μm are combined, is preferred, since blending irregularity ishard to occur when both the kinds of fibers are blended for use.Furthermore, a filter fabric composed of fibers with a fiber diameter ofabout 15 μm can be so constituted as to especially achieve a goodbalance between the strength and the denseness in the filter fabric. So,it can be suitably used for a filter. The balance between the strengthand the denseness of a filter fabric is explained below. A filter fabriccomposed of fibers with a fiber diameter of 9 μm or smaller only is notpreferred, since the mechanical strength, especially burst strength ofthe filter fabric declines, though the efficiency of collecting dustwith particle sizes of 0.5 μm and less can be enhanced since the poresize of the filter fabric becomes small. Furthermore, in the case of afilter fabric composed of fibers with a fiber diameter of 9 μm, orsmaller only, it can be considered to increase the weight per unit areaof the filter fabric for securing the mechanical strength. However, thismethod is not preferred, since in the case where the weight per unitarea of the filter fabric is set, for example, at 900 g/m² or more, theapparent density of the nonwoven fabric becomes very high to lower theair permeability and to raise the pressure loss. On the contrary, afilter fabric composed of fibers with a fiber diameter of 17 μm orlarger only is not preferred either, since the pore size of the filterfabric becomes so large as to lower the efficiency of collecting dustwith particle sizes of 0.5 μm and less, though the burst strength can beenhanced. Therefore, a filter fabric composed of fibers with the fiberdiameter kept in a range from about 9 to about 15 μm is preferred, sinceit is excellent in the balance between the strength and the denseness inthe nonwoven fabric.

For conversion between the fineness of a fiber and the diameter of thefiber, if the fiber has a round cross-sectional form, the specificweight is used for calculation. If the fiber has an irregularcross-sectional form, the mean value of the distances from the center ofgravity to the respective vertexes (including acute and obtuse internalangles) of the polygon in a polygonal cross-sectional form of the fiberis defined as the diameter of the fiber. In the case of an ellipsoidalfiber, the mean value of the major axis and the minor axis is defined asthe diameter of the fiber. The specific weight employed for PPS fibersis 1.34, and the specific weight employed for fluorine-based fibers,2.30.

Therefore, the diameter of PPS staple fibers with their fineness kept ina range from 1 to 3 dtex is from 9.7 to 16.9 μm, and the diameter offluorine-based staple fibers with their fineness kept in a range from 2to 4 dtex is from 10.5 to 14.9 μm.

In the filter fabric of this invention, it is preferred that PPS staplefibers and fluorine-based staple fibers are blended to form a nonwovenfabric. It is preferred that the fiber length of the PPS staple fibersand the fluorine-based staple fibers is in a range from 0.2 to 140 mm.It is not preferred that the fibers are longer than 140 mm, since theblending of fibers is insufficient, not allowing the fabric uniformityto be uniform. Especially in the case where fibers are blended using anopener, it is more suitable that the fiber length is in a range from 35to 80 mm. In the case where the fibers are blended while they aredispersed in water, it is more suitable that the fiber length is in arange from 0.2 to 10 mm. It is not preferred that the fibers have afiber length of less than 0.2 mm, for such reasons that the lengths ofthe fibers cut in the cutting step become irregular, and that since thefibers very often adhere to the blade in the cutting step, they do notpass through the step smoothly.

In this invention, it is also preferred that the filter fabric containsa woven fabric composed of heat resistant fibers, namely, scrim. Atwo-layer structure having a web laminated on one surface of the scrimcan also be used, but at least a three-layer structure having webslaminated on both the surfaces of the scrim is especially preferred.

The filter fabric of this invention can be produced, for example, by amethod comprising the steps of blending PPS staple fibers with afineness of 1 to 3 dtex and fluorine-based staple fibers with a finenessof 2 to 4 dtex, passing the blended fibers through a carding machine, toform a web, laminating it on an scrim, and entangling and integratingthe laminate by needle punching treatment. The blending in this case canbe blending using a general opener, or blending by means of aeoliantransport can also be used without any problem.

In this invention, as the PPS fibers, fluorine-based fibers, scrim,etc., those enumerated before can be suitably used. Furthermore, also asthe web entangling method, etc., the above-mentioned methods can besuitably used.

As the filter fabric of this invention, a filter fabric having amicroporous polytetrafluoroethylene film laminated on it can also besuitably used. Laminating and bonding a microporouspolytetrafluoroethylene film is preferred, since the efficiency forcollecting fine dust can be enhanced. Laminating a microporouspolytetrafluoroethylene film on a filter fabric composed ofpolytetrafluoroethylene fibers has a disadvantage that adhesiveness israther poor in view of the nature of the polymer. Since the filterfabric of this invention is higher in adhesiveness than a filter fabriccomposed of polytetrafluoroethylene fibers only, because of the presenceof PPS fibers, it also has an effect that lamination is easier.

The filter fabric obtained like this can be sewn into a bag, and can besuitably used as a bag filter in need of heat resistance, for collectingthe exhaust gas of a refuse incinerator, coal boiler, metal blastfurnace or the like. It is preferred that the sewing threads used forthe sewing are yarns made of the same material having chemicalresistance and heat resistance as that of the fibers constituting thewoven fabric, and PPS fibers, fluorine-based fibers or the like can beadequately used.

This invention is explained below in more detail in reference toexamples, but is not limited thereto or thereby.

Meanwhile, the methods for measuring the respective physical propertiesof filter fabrics are as follows.

[Weight Per Unit Area]

A filter fabric was cut into a 400 mm square, and the weight per unitarea of the filter fabric was calculated from its weight.

[Thickness]

The thickness of a filter fabric was measured using a thickness dialgauge (pressing pressure 250 g/cm²=0.000245 Pa). Measurement was made atsix places selected at random, and the mean value was obtained.

[Stiffness]

The stiffness of a filter fabric was measured based on the Gurley methodspecified in JIS L 1096. The filter fabric was cut to have a length of63.5 mm and a width of 25.4 mm, and measurement was made once each onthe front and back surfaces. Four samples were measured.

[Burst Strength]

The burst strength of a filter fabric was measured based on the burststrength method specified in JIS L 1096. Measurement was made at fiveplaces selected at random.

[Air Permeability]

The air permeability of a filter fabric was measured based on theFrazier method specified in JIS L 1096. Measurement was made at sixplaces selected at random.

[Dust-Collecting Efficiency for Dust in the Air]

The dust-collecting efficiency of a filter fabric was measured by amethod of counting the particles of dust in the air using the instrumentof FIG. 2. That is, in FIG. 2, an air stream with a filtration airvelocity of 1 m/min was made to pass through a filter fabric 5 (170 mmdiameter) for 5 minutes by an air blower 8 installed downstream of thefilter fabric 5, and the number A of particles of dust (particle size0.3 to 5 μm) in the air upstream of the filter fabric 5 was countedusing a particle counter (upstream) 4 produced by RION. At the sametime, the number B of particles of dust (particle size 0.3 to 5 μm) inthe air downstream of the filter fabric 5 was couted by a particlecounter (downstream) 6 produced by the same company. Three samples weremeasured. From the obtained results of measurement, the collectingefficiency (%) was obtained from the following formula:(1−(B/A))×100

where A: Number of particles of dust in the upstream air

-   -   B: Number of particles of dust in the downstream air        The dust-collecting efficiency for dust in the air was judged        according to the following criterion:

-   o: The efficiency of collecting dust with particle sizes of 1 μm and    less is 50% or more (good)

-   Δ: Said efficiency is from 45% to less than 50% (rather good)

-   x: Said efficiency is less than 45% (poor)    [Pressure Loss]

The pressure loss by a filter fabric 5 during the measurement ofdust-collecting efficiency for dust in the air was read using amanometer 7.

[Pressure Loss after Pulse Cleaning]

The apparatus of FIG. 3 was used to measure the pressure loss afterpulse cleaning. That is, in FIG. 3, an air stream with a filtration airvelocity of 2.0 m/min was given to a filter fabric 5 (170 mm diameter)using a vacuum pump 12 and a flow meter 10 installed downstream of thefilter fabric 5. JIS Class 10 dust was adjusted to a dust concentrationof 20 g/m³ using a dust feeder 14 and a dust dispersing device 15, andit was applied to the dust side of the filter fabric 5 (filtration area100 cm²). Whenever the pressure loss measured by a digital manometer 13rose to 100 mm H₂O (980 Pa), 155 jet pulses were applied at a pulse jetpressure of 3 kgf/cm² (294 kPa) for 0.1 second by a pulse jet loadingmachine 9 installed downstream of the filter fabric 5, and the pressureloss immediately after pulse jet application was continuously monitoredby a digital manometer 13.

The pressure loss after pulse cleaning was judged according to thefollowing criterion:

-   o: Pressure loss after lapse of 30 hours was less than 7 mm H₂O (69    Pa)(good).-   Δ: Said pressure loss was from 7 mm H₂O(69 Pa) to 8 mm H₂O (78 Pa)    (rather good).-   x: Said pressure loss was more than 8 mm H₂O (78 Pa) (poor)    [Overall Judgment]

The overall judgment was made according to the following criterion:

-   x: At least either the dust collecting-efficiency for dust in the    air or the pressure loss after pulse cleaning was judged as x.-   o: Both the dust-collecting efficiency for dust in the air and the    pressure loss after pulse cleaning were judged as either o or Δ.    [Strength of Filter Fabric]

The tensile strength of a filter fabric was measured based on the stripmethod specified in JIS L 1096. The sample size was 200 mm×50 mm, andthe tensile-strength was measured at a stress rate of 100 mm/min at achuck interval of 100 mm. The measured value was the value of breakingstrength in the length direction of the sample (the directionperpendicular to the orientation of the fibers, or the same direction asthe warp direction of a scrim if the sample contains the scrim).

[Heat Resistance (Hydrolytic Resistance)]

A sample was treated under the following conditions to obtain thebreaking strength retention rate in the length direction of the sample(the direction perpendicular to the orientation of the fibers, or thesame direction as the warp direction of a scrim if the sample containsthe scrim). The breaking strength retention rate was obtained from thefollowing formula:Breaking strength retention rate (%)=A/B×100

-   A: Breaking strength of the sample treated in an autoclave-   B: Breaking strength of the sample not yet treated in an autoclave

The treatment was carried out using an autoclave (produced by NittoAutoclave) under the following conditions: set temperature 160° C.,indicated pressure 6.5 kgf/cm² (637 kPa) for 200 hours.

[Dimensional Stability]

A sample was cut to have a size of 300 mm (length)×50 mm (width), and aload of 9.8 N was suspended from the sample in the vertical direction insuch a manner that the length direction of the sample agreed with thevertical direction, and it was kept in an atmosphere of 240° C. for 1hour. After completion of said heat treatment, the elongation (creep) inthe length direction of the sample was measured. The formula forobtaining the creep was as follows. A creep closer to zero in absolutevalue indicates better dimensional stability.Creep (%)={(Length of sample after heat treatment)−300}×100/300[Fabric Uniformity]

A sample was held against light to visually judge the fabric uniformityand the presence or absence of pinholes and to evaluate according to thefollowing criterion:

-   o: Good-   Δ: Rather good-   x: Poor

EXAMPLE 1

PPS staple fibers with a fineness of 3.0 dtex (fiber diameter 17 μm) anda cut length of 76 mm (“TORCON®” S101-3.0T76 mm, produced by TorayIndustries, Inc.) were used to obtain a spun yarn (total fineness 600dtex) having a single yarn count of 20 s obtained by doubling two yarns.Yarns, each produced as above, were used to form a PPS spun yarn plainweave fabric with a warp density of 26 threads/2.54 cm and a weftdensity of 18 threads/2.54 cm. The woven fabric was used as the scrim.Meanwhile, PPS staple fibers with a fineness of 2.2 dtex (fiber diameter14.5 μm) and a cut length of 51 mm (“TORCON®” S101-2.2T51 mm, producedby Toray Industries, Inc.) and PPS staple fibers with a fineness of 1.0dtex (fiber diameter 9.7 μm) and a cut length of 51 mm (“TORCON®”S101-1.0T51 mm, produced by Toray Industries, Inc.) were blended at aratio by weight of 50:50, and the blended staple fibers were treatedusing an opener and a carding machine, and temporarily needle-punched ata needling density of 50 needles/cm², to obtain a web. The web waslaminated on one surface of the scrim at a weight per unit area of 194g/m². The web formed the filtration layer on the dust side. On the otherhand, PPS staple fibers with a fineness of 7.8 dtex (fiber diameter 27.2μm) and a cut length of 51 mm (“TORCON®” S101-7.8T51 mm, produced byToray Industries, Inc.) only were treated using an opener and a cardingmachine and temporarily needle-punched at a needling density of 50needles/cm², to obtain a web. The web was laminated on the other surfaceof the woven fabric at a weight per unit area of 220 g/m². The webformed the filtration layer on the clean side. Furthermore, the laminatewas needle-punched to entangle the woven fabric (scrim) and theabove-mentioned webs, to obtain a filter fabric with a weight per unitarea of 544 g/m² and a total needling density of 300 needles/cm². Theperformance of the obtained filter fabric is shown in Table 1 and FIGS.4 and 5. The filter fabric obtained here tended to have a weight perunit area higher than a theoretical value since it was contracted by theneedle punching treatment.

EXAMPLE 2

PPS staple fibers with a fineness of 1.0 dtex (fiber diameter 9.7 μm)and a cut length of 51 mm, PPS staple fibers with a fineness of 2.2 dtex(fiber diameter 14.5 μm) and a cut length of 51 mm, and PPS staplefibers with a fineness of 7.8 dtex (fiber diameter 27.2 μm) and a cutlength of 51 mm were blended at 30:30:40, for being used as the fibersconstituting the web on the dust side. Furthermore, PPS staple fiberswith a fineness of 2.2 dtex and a cut length of 51 mm and PPS staplefibers with a fineness of 7.8 dtex and a cut length of 51 mm wereblended at 50:50 for being used as the fibers constituting the web onthe clean side. A filter fabric was obtained by the same method asdescribed for Example 1, except that the above webs were used. Theperformance of the obtained filter fabric is shown in Table 1 and FIGS.4 and 5.

EXAMPLE 3

A filter fabric was obtained by the same method as described for Example1, except that PPS staple fibers with a fineness of 1.0 dtex (fiberdiameter 9.7 μm) and a cut length of 51 mm and PPS staple fibers with afineness of 2.2 dtex (fiber diameter 14.5 μm) and a cut length of 51 mmwere blended for being used as the fibers constituting the web on thedust side. The performance of the obtained filter fabric is shown inTable 1 and FIGS. 4 and 5.

EXAMPLE 4

PPS staple fibers with a fineness of 2.2 dtex (fiber diameter 14.5 μm)and a cut length of 51 mm (“TORCON®” S101-2.2T51 mm, produced by TorayIndustries, Inc.) and PTFE staple fibers with a fineness of 3.3 dtex(fiber diameter 13.5 μm) and a cut length of 70 mm (“TOYOFLON®”T201-3.3T70 mm, produced by Toray Fine Chemicals Co., Ltd.) were blendedat a ratio by weight of 50:50, and the blended staple fibers weretreated using an opener and a carding machine, and temporarilyneedle-punched at a needling density of 40 needles/cm², to obtain 210g/m² of a web forming the filtration layer on the dust side. A filterfabric with a weight per unit area of 533 g/m² was produced by the samemethod as described for Example 1, except that the above web was used.The performance of the obtained filter fabric is shown in Table 1 andFIGS. 4 and 5.

EXAMPLE 5

PPS staple fibers with a fineness of 2.2 dtex (fiber diameter 14.5 μm)and a cut length of 51 mm (“TORCON®” S101-2.2T51 mm, produced by TorayIndustries, Inc.) only were treated using an opener and a cardingmachine, and temporarily needle-punched at a needling density of 40needles/cm², to obtain 220 g/m² of a web forming the filtration layer onthe dust side. A filter fabric with a weight per unit area of 494 g/m²was obtained using PPS staple fibers with a fineness of 7.8 dtex(“TORCON®” S101-7.8T51 mm, produced by Toray Industries, Inc.) by thesame method as described for Example 1, except that the above-mentionedweb was used. The performance of the obtained filter fabric is shown inTable 1 and FIGS. 4 and 5.

EXAMPLE 6

PPS fibers obtained by a melt spinning method were heat-treated undertension in their drawing step to obtain PPS staple fibers with a Young'smodulus of 28 cN/dtex, a fineness of 2.2 dtex (fiber diameter 14.5 μm)and a cut length of 76 mm obtained by an emulsion spinning method(TORCON® produced by Toray Industries, Inc.). The PPS staple fibers andpolytetrafluoroethylene staple fibers with a fineness of 3.3 dtex (fiberdiameter 13.5 μm) and a cut length of 70 mm (TOYOFLON® produced by TorayIndustries, Inc.) were blended at a ratio by weight of 50:50 using anopener, and the blended fibers were fed through a carding machine, toform a sheet-like web. PPS staple fibers with a Young's modulus of 28cN/dtex, a fineness of 2.2 dtex and a cut length of 76 mm (TORCON®produced by Toray Industries, Inc.) were used to form a spun yarn havinga total single yarn count of 20 s obtained by doubling two yarns (totalfineness 600 dtex), and yarns, each produced as above, were used to forma plain weave fabric (#2818, warp density 28 threads/2.54 cm, weftdensity 18 threads/2.54 cm). This woven fabric was used as the scrim.The web and the scrim were laminated in the order of web/scrim/web, andthe laminate was needle-punched at 400 needles/cm², to obtain a felt.The felt was pressed using a hot press roll machine with iron rolls at100° C. with a clearance of 1.0 mm, to obtain a filter fabric with aweight per unit area of 715 g/m² and a thickness of 1.42 mm.

The obtained filter fabric was uniform in the fabric uniformity and hadfew pinholes, being a nonwoven fabric having PPS fibers andpolytetrafluoroethylene fibers blended homogeneously.

EXAMPLE 7

A filter fabric with a weight per unit area of 703 g/m² and a thicknessof 1.58 mm were produced by the same method as described for Example 6,except that PPS staple fibers and polytetrafluoroethylene staple fibersused for the sheet-like web were blended at a ratio by weight of 75:25using an opener. The obtained filter fabric was uniform in the fabricuniformity and had few pinholes, being a nonwoven fabric having PPSfibers and polytetrafluoroethylene fibers blended homogeneously.

EXAMPLE 8

As the polytetrafluoroethylene staple fibers used for a sheet-like web,PROFILEN® type 803/60 with a fineness of 2.7 dtex (fiber diameter 12.2μm) and a cut length of 60 mm, produced by Lenzing, were used.Furthermore, the PPS staple fibers used for the sheet-like web ofExample 6 and said polytetrafluoroethylene staple fibers were blended ata ratio by weight of 25:75 using an opener, to form a sheet-like web.Furthermore, Rastex® Scrim (plain weave fabric using PTFE slit yarns 400D, having a weaving density of 20 threads/2.54 cm and a weight per unitarea of 70 g/m², produced by Japan GORE-TEX Inc.) was used as the scrim.A filter fabric with a weight per unit area of 722 g/m² and a thicknessof 1.29 mm was obtained by the same method as described for Example 6,except the above-mentioned sheet-like web and scrim were used. Theobtained filter fabric was uniform in the fabric uniformity and had fewpinholes, being a nonwoven fabric having PPS fibers andpolytetrafluoroethylene fibers blended homogeneously. However, the fluffon the surface was somewhat outstanding.

EXAMPLE 9

A filter fabric with a weight per unit area of 408 g/m² was obtained bythe same method as described for Example 1, except that a two-layerstructure consisting of the filtration layer on the dust side and thefiltration layer on the clean side was employed without using the scrim.The performance of the obtained filter fabric is shown in Table 1 andFIGS. 4 and 5.

EXAMPLE 10

A filter fabric with a weight per unit area of 413 g/m² was obtained bythe same method as described for Example 4, except that a two-layerstructure consisting of the filtration layer on the dust side and thefiltration layer on the clean side was employed without using the scrim.The performance of the obtained filter fabric is shown in Table 1 andFIGS. 4 and 5.

COMPARATIVE EXAMPLE 1

PPS staple fibers with a fineness of 2.2 dtex (fiber diameter 14.5 μm)and a cut length of 51 mm (“TORCON® ”S101-2.2T51 mm, produced by TorayIndustries, Inc.) only were treated using an opener and a cardingmachine, and temporarily needle-punched at a needling density of 40needles/cm², to obtain a web with a weight per unit area of 220 g/m²used to form the filtration layer on the dust side and a web with aweight per unit area of 220 g/m² used to form the filtration layer onthe clean side. A filter fabric with a weight) per unit area of 571 g/m²was obtained by the same method as described for Example 1, except thatthe above-mentioned webs were used. The performance of the obtainedfilter fabric is shown in Table 1 and FIGS. 4 and 5.

COMPARATIVE EXAMPLE 2

PPS staple fibers with a fineness of 7.8 dtex and a cut length of 51 mm(“TORCON®”S101-7.8T51 mm, produced by Toray Industries, Inc.) only weretreated using an opener and a carding machine, and temporarilyneedle-punched at a needling density of 40 needles/cm², to obtain a webwith a weight per unit area of 225 g/m² used to form the filtrationlayer on the dust side and a web with a weight per unit area of 228 g/m²used to form the filtration layer on the clean side. A filter fabricwith a weight per unit area of 594 g/m² was obtained by the same methodas described for Example 1, except that the above-mentioned webs wereused. The performance of the obtained filter fabric is shown in Table 1and FIGS. 4 and 5.

COMPARATIVE EXAMPLE 3

PPS staple fibers obtained by a melt spinning method were heat-treatedunder relaxation in their drawing step, to obtain PPS staple fibers witha Young's modulus of 19 cN/dtex, a fineness of 7.8 dtex (fiber diameter27.7 μm) and a cut length of 76 mm (TORCON®, produced by TorayIndustries, Inc.). The PPS staple fibers were treated using an openerand a carding machine, to form a sheet-like web. As for the scrim, PPSstaple fibers with a Young's modulus of 19 cN/dtex (TORCON® with afineness of 2.2 dtex and a cut length of 76 mm, produced by TorayIndustries, Inc.) were used to form a spun yarn (total fineness 600dtex) having a single yarn count of 20s obtained by doubling two yarns,and yarns, each produced as above, were used to form a plain weavefabric (#2818, warp density 28 threads/2.54 cm and weft density 18threads/2.54 cm). The web and the scrim were laminated in the order ofweb/scrim/web, and the laminate was needle-punched at 400 needles/cm²,to obtain a felt. The felt was pressed using a hot press roll machinewith iron rolls at 100° C. with a clearance of 1.0 mm, to obtain afilter fabric with a weight per unit area of 551 g/m² and a thickness of1.52 mm.

The obtained filter fabric was a nonwoven fabric uniform in the fabricuniformity but marked with needle holes of the needle punch.

COMPARATIVE EXAMPLE 4

A filter fabric with a weight per unit area of 630 g/m² and a thicknessof 1.25 mm was obtained by the same method as described for ComparativeExample 2, except that polytetrafluoroethylene staple fibers with afineness of 7.4 dtex and a cut length of 70 mm obtained by an emulsionspinning method (TOYOFLON® produced by Toray Industries, Inc.) were usedas the sheet-like web, and that Rastex® Scrim (plain weave fabric with aweaving density of 20 threads/2.54 cm and a weight per unit area of 70g/m² using PTFE slit yarns 400 D, produced by Japan GORE-TEX Inc.) wasused as the scrim.

The obtained filter fabric was a nonwoven fabric uniform in the fabricuniformity but marked with needle holes of the needle punch.

COMPARATIVE EXAMPLE 5

A filter fabric with a weight per unit area of 558 g/m² and a thicknessof 1.53 mm was obtained by the same method as described for ComparativeExample 1, except that polyethylene terephthalate staple fibers with afineness of 2.2 dtex and a cut length of 51 mm (TETORON® produced byToray Industries, Inc.) were used as the sheet-like web and that noscrim was used.

The obtained filter fabric was a nonwoven fabric uniform in the fabricuniformity but marked with needle holes of the needle punch.

COMPARATIVE EXAMPLE 6

PPS fibers obtained by a melt spinning method were heat-treated underrelaxation in their drawing step, to obtain PPS staple fibers with aYoung's modulus of 19 cN/dtex, a fineness of 2.2 dtex and a cut lengthof 76 mm (TORCON® produced by Toray Industries, Inc.). The PPS staplefibers were treated using an opener and a carding machine, to form asheet-like web. A filter fabric with a weight per unit area of 512 g/m²and a thickness of 1.96 mm was obtained by the same method as describedfor Comparative Example 2, without using the scrim.

The obtained filter fabric was a nonwoven fabric somewhat non-uniform inthe fabric uniformity. TABLE 1 Sample Example 1 Example 2 Example 3Example 4 Example 5 Web Dust PPS 1.0 T (9.7 μm 28 cN/dtex) 50 wt % 30 20constitution side PPS 2.2 T (14.5 μm 28 cN/dtex) PPS 2.2 T (14.5 μm 19cN/dtex) 50 wt % 30 80 50 100 PPS 7.8 T (27.2 μm 19 cN/dtex) 40 Fluorine3.3 T (13.5 mm) 50 Fluorine 7.4 T (20.2 μm) Fluorine 2.7 T (12.2 μm) PET2.2 T (14.2 μm) glass 0.3 T (6 μm) Clean PPS 2.2 T (14.5 μm 28 cN/dtex)side PPS 2.2 T (14.5 μm 19 cN/dtex) 50 PPS 7.8 T (27.2 μm 19 cN/dtex)100 wt % 50 100 100 100 Fluorine 3.3 T (13.5 μm) Fluorine 7.4 T (20.2μm) Fluorine 2.7 T (12.2 μm) PET 2.2 T (14.2 μm) glass 0.3 T (6 μm)Scrim PPS 2.2 T (Young's modulus 28 cN/dtex) constitution PPS 2.2 T(Young's modulus 19 cN/dtex) Used Used Used Used Used PTFE Rastex Weightper unit area (g/m²) 544 528 540 533 494 Thickness (mm) 2.7 2.6 2.6 2.42.4 Apparent density (g/cm³) 0.20 0.20 0.21 0.22 0.21 Stiffness (mN) 6365 69 71 69 Burst strength (kPa) 4200 4160 4380 4440 4540 Airpermeability (cc/cm² · sec) 39.8 43 45 35 46.4 Pressure loss (mm H₂O at1 m/min) 0.46 0.48 0.47 0.46 0.48 (mm H₂O at 2 m/min) 0.89 0.95 0.92 0.90.85 Collection efficiency (%) ∘ Δ Δ ∘ Δ Pressure loss after pulsecleaning Δ ∘ ∘ Δ ∘ Overall judgment ∘ ∘ ∘ ∘ ∘ Strength (tensile) (N/5cm) — — — — — Heat resistance (Hydrolytic resistance) (%) — — — — —Dimensional stability (%) — — — — — Fabric uniformity — — — — — SampleExample 6 Example 7 Example 8 Example 9 Example 10 Web Dust PPS 1.0 T(9.7 μm 28 cN/dtex) 50 constitution side PPS 2.2 T (14.5 μm 28 cN/dtex)50 75 25 PPS 2.2 T (14.5 μm 19 cN/dtex) 50 50 PPS 7.8 T (27.2 μm 19cN/dtex) Fluorine 3.3 T (13.5 mm) 50 25 50 Fluorine 7.4 T (20.2 μm)Fluorine 2.7 T (12.2 μm) 75 PET 2.2 T (14.2 μm) glass 0.3 T (6 μm) CleanPPS 2.2 T (14.5 μm 28 cN/dtex) 25 side PPS 2.2 T (14.5 μm 19 cN/dtex) 5075 PPS 7.8 T (27.2 μm 19 cN/dtex) 100 100 Fluorine 3.3 T (13.5 μm) 50 25Fluorine 7.4 T (20.2 μm) Fluorine 2.7 T (12.2 μm) 75 PET 2.2 T (14.2 μm)glass 0.3 T (6 μm) Scrim PPS 2.2 T (Young's modulus 28 cN/dtex) UsedUsed constitution PPS 2.2 T (Young's modulus 19 cN/dtex) PTFE RastexUsed Weight per unit area (g/m²) 715 703 722 408 413 Thickness (mm) 1.421.58 1.29 2.2 1.9 Apparent density (g/cm³) 0.50 0.44 0.56 0.19 0.22Stiffness (mN) — — — 44 50 Burst strength (kPa) — — — 3370 3550 Airpermeability (cc/cm² · sec) 14.7 15.2 13.8 47 42 Pressure loss (mm H₂Oat 1 m/min) — — — 0.55 0.51 (mm H₂O at 2 m/min) — — — 1.09 1.01Collection efficiency (%) 63.2 61.2 62 Δ Δ Pressure loss after pulsecleaning — — — ∘ ∘ Overall judgment — — — ∘ ∘ Strength (tensile) (N/5cm) 862 892 824 — — Heat resistance (Hydrolytic resistance) (%) 102 102105 — — Dimensional stability (%) 0.9 1.1 0.4 — — Fabric uniformity ∘ ∘∘ — — Com- Com- Com- Com- Com- Com- parative parative parative parativeparative parative Example Example Example Example Example Example Sample1 2 3 4 5 6 Web Dust PPS 1.0 T (9.7 μm 28 cN/dtex) constitution side PPS2.2 T (14.5 μm 28 cN/dtex) PPS 2.2 T (14.5 μm 19 cN/dtex) 100 100 PPS7.8 T (27.2 μm 19 cN/dtex) 100 100 Fluorine 3.3 T (13.5 mm) Fluorine 7.4T (20.2 μm) 100 Fluorine 2.7 T (12.2 μm) PET 2.2 T (14.2 μm) 100 glass0.3 T (6 μm) Clean PPS 2.2 T (14.5 μm 28 cN/dtex) side PPS 2.2 T (14.5μm 19 cN/dtex) 100 100 PPS 7.8 T (27.2 μm 19 cN/dtex) 100 100 Fluorine3.3 T (13.5 μm) Fluorine 7.4 T (20.2 μm) 100 Fluorine 2.7 T (12.2 μm)PET 2.2 T (14.2 μm) 100 glass 0.3 T (6 μm) Scrim PPS 2.2 T (Young'smodulus 28 cN/dtex) constitution PPS 2.2 T (Young's modulus 19 cN/dtex)Used Used Used PTFE Rastex Used Weight per unit area (g/m²) 571 594 551630 558 512 Thickness (mm) 2.7 2.9 1.52 1.25 1.53 1.96 Apparent density(g/cm³) 0.21 0.20 0.36 0.50 0.37 0.26 Stiffness (mN) 53 88 — — — — Burststrength (kPa) 2900 6120 — — — — Air permeability (cc/cm² · sec) 30 59.919 23.9 11.5 14.8 Pressure loss (mm H₂O at 1 m/min) 0.79 0.31 — — — —(mm H₂O at 2 m/min) 1.52 0.6 — — — — Collection efficiency (%) x x 46.148.2 50.7 48.2 Pressure loss after pulse cleaning x ∘ — — — — Overalljudgment x x — — — — Strength (tensile) (N/5 cm) — — 970 796 487 443Heat resistance (Hydrolytic resistance) (%) — — 102 104 Could not 103 beDimensional stability (%) — — 2.8 0.4 5.8 3.3 Fabric uniformity — — Δ ΔΔ Δ

From the evaluation results of Table 1 and FIGS. 4 and 5, it can be seenthat the filter fabrics of Examples 1 to 5 are high in dust-collectingefficiency and also high in burst strength and Gurley stiffness comparedwith the filter fabrics of Comparative Examples 1 and 2, and also gentlein the rise of pressure loss after pulse cleaning. Since the filterfabrics are excellent in mechanical strength, they are strong againstthe stress acting during pulse cleaning and also against the wear withthe cage, and do not become high in pressure loss even if they arecontinuously used with intermittent pulse cleaning. So, the filters canbe elongated in life. Furthermore, as can be seen from Examples 9 and10, the filter fabrics of this invention have sufficient mechanicalstrength even though they do not contain any scrim. Moreover, it can beseen that the filter fabrics of Examples 6 to 8 are high indust-collecting efficiency, high in hydrolytic resistance and high indimensional stability at high temperature and uniform in the fabricuniformity and have few pinholes, compared with the filter fabrics ofComparative Examples 3 to 6.

INDUSTRIAL APPLICABILITY

This invention provides, a filter fabric containing polyphenylenesulfide fibers, comprising at least two webs, one of which on the dustside contains 50 wt % or more of heat resistant fibers with a fiberdiameter of 15 μm or less, while the other web on the clean sidecontains 50 wt % or more of heat resistant fibers with a fiber diameterof 20 μm or more. So, the filter fabric is, excellent in dust-collectingefficiency, small in the rise of pressure loss after pulse cleaning andexcellent in mechanical strength.

Furthermore, this invention provides a filter fabric containingpolyphenylene sulfide fibers, comprising a web containing polyphenylenesulfide staple fibers with their fineness kept in a range from 1 to 3dtex and fluorine-based staple fibers with their fineness kept in arange from 2 to 4 dtex. So, the filter fabric is excellent indust-collecting efficiency, excellent in thermal dimensional stabilityat high temperature and uniform in the fabric uniformity, and has fewdefects such as pinholes.

Furthermore, since the bag filter of this invention comprises saidfilter fabric, it is excellent in the efficiency of collecting the dustcontained in an exhaust gas, and high and excellent in mechanicalstrength such as abrasion resistance to the cage, burst strength,stiffness and dimensional stability. So, the life of the filter can beelongated.

The filter fabric or the bag filter of this invention can be suitablyused as the dust collecting filter cloth or as the bag filter composedof the filter cloth, for filtering especially the high temperatureexhaust gas discharged from a refuse incinerator, coal boiler, metalblast furnace or the like.

1. A filter fabric containing polyphenylene sulfide fibers,characterized in that the following (1) and/or (2) is satisfied: (1)Containing at least two webs, one of which on the dust side contains 50wt % or more of heat resistant fibers with a fiber diameter of 15 μm orless, while the other web on the clean side contains 50 wt % or more ofheat resistant fibers with a fiber diameter of 20 μm or more (2)Containing a web containing polyphenylene sulfide staple fibers withtheir fineness kept in a range from 1 to 3 dtex and fluorine-basedstaple fibers with their fineness kept in a range from 2 to 4 dtex.
 2. Afilter fabric, according to claim 1, which contains at least two webs,one of which on the dust side contains 50 wt % or more of heat resistantfibers with a fiber diameter of 15 μm or less, while the other web onthe clean side contains 50 wt % or more of heat resistant fibers with afiber diameter of 20 μm or more.
 3. A filter fabric, according to claim1, wherein the web on the dust side contains 20 wt % or more of heatresistant fibers with a fiber diameter of 10 μm or less.
 4. A filterfabric, according to claim 1, wherein the heat resistant fibers areselected from para-aramid fibers, meta-aramid fibers, polyphenylenesulfide fibers, polyimide fibers, fluorine-based fibers, carbon fibersand glass fibers.
 5. A filter fabric, according to of claim 1, whereinthe web on the dust side contains 10 to 90 wt % of polyphenylene sulfidefibers and 10 to 90 wt % of fluorine-based fibers.
 6. A filter fabric,according to claim 1, which comprises a web containing polyphenylenesulfide staple fibers with their fineness kept in a range from 1 to 3dtex and fluorine-based staple fibers with their fineness kept in arange from 2 to 4 dtex.
 7. A filter fabric, according to claim 1,wherein polyphenylene sulfide staple fibers and fluorine-based staplefibers are blended to form a web.
 8. A filter fabric, according to claim1, wherein the total rate of polyphenylene sulfide fibers andfluorine-based fibers in a web as a whole is 70 wt % or more.
 9. Afilter fabric, according to claim 1, wherein the fibers on the surfaceof a web are partially fused.
 10. A filter fabric, according to claim 1,wherein webs are laminated on both sides of a scrim composed of heatresistant fibers to form at least a three-layer structure.
 11. A filterfabric, according to claim 10, wherein the scrim is a woven fabriccomposed of heat resistant fibers.
 12. A filter fabric, according toclaim 10, wherein the scrim contains fibers selected from polyphenylenesulfide fibers and fluorine-based fibers.
 13. A filter fabric, accordingto claim 10, wherein the heat resistant fibers constituting the scrimare spun polyphenylene sulfide yarns, the total fineness of which iskept in range from 100 to 1000 dtex.
 14. A filter fabric, according toclaim 10, wherein the Young's modulus of the polyphenylene sulfidefibers is 20 cN/dtex or more.
 15. A filter fabric, according to claim10, wherein the fluorine-based fibers are polytetrafluoroethylenefibers.
 16. A bag filter, produced by sewing the filter fabric as setforth in claim 1 as a bag.